It is known that electromagnetic pumps are employed for adding liquids, such as detergents, sanitisers, and disinfectants, to aqueous solutions through a predeterminable dosage repeatable in time.
In particular, the liquid is dosed into the solution through the mechanical action of an interposition membrane, moved by the action of two opposed forces: a pushing force, obtained through the magnetic attraction exerted on a ferromagnetic piston by an electromagnet, suitably driven by an electronic control circuit, and a return force, obtained through the repulsive action of a spring coaxial with the piston that is loaded by the same piston during the pushing phase. During the operation, the electromagnet is operated by an electric current and it pushes the piston into the pump body, so that, through suitable valves, the liquid to dose is let into the hydraulic circuit; the piston is then brought back to rest by the spring loaded during the active pushing phase.
At each injection or stroke a certain quantity of liquid is let into the hydraulic circuit, whereby multiplying it by the number of injections (strokes) per minute the capacity in the time unit is obtained, usually measured in liters/hour.
Presently, for reasons of adaptability to the hydraulic equipments where they are installed, dosing electromagnetic pumps need adjustments defining the capacity thereof as a function of the operating pressure and of the dosed liquid viscosity.
Therefore all the manufacturing companies give as adjustments the number of stroke/minute (conventionally indicated in liters/hour) necessary for obtaining a certain capacity: by adjusting the number of strokes/minute the quantity of liquid let into the hydraulic circuit in the time unit may be varied.
However, such solution suffers from the drawback that the reduction of the number of strokes, which all arrive to the piston stop, produces as a consequence that dosage concentrations are significantly non uniform in the time unit.
Some alternative solutions comprise a device that is provided with a further adjustment of the piston stroke through mechanical means, by limiting the movement of the cap, operated by the piston, that in turn moves the membranes. In particular, such solutions provide that the origin of the stroke is moved towards the stop limit (i.e., it is moved forward), whereby the volume of the injected liquid is directly proportional to the residual movement of the piston.
However, even these solutions have great functionality limitations, due to the fact that the pump capability of priming the liquid to dose contained in a suitable tank is greatly limited.
Further solutions use sensors for detecting the movement of the piston, and hence of the moving cap.
However, such solutions suffer from the drawback the adjusting device is extremely complex and expensive, since it needs very sophisticated and extremely precise sensors, considering that travels at stake are typically of the order of one or two millimeters, and they are hardly installable within the pump, due to the reduced room within the same.
Moreover, it is however left the main problem of the large variation which capacity undergoes as the operating pressure applied outside the pump by the hydraulic circuit into which the liquid to dose is to be let varies. In fact, for functional reasons of hydraulic nature, the membrane pushing the liquid must have elastic characteristics which are then the cause of the capacity variations as pressure varies. Such elastic coefficient is determined by the geometric shape and by the material of which the membrane is made, typically Teflon.